The U.S. Supreme Court has refused to hear the case of an accused murderer who claims he has a constitutional right to an insanity defense.

Justice Stephen G. Breyer dissented to the cert denial in an opinion (PDF) joined by Justices Ruth Bader Ginsburg and Sonia Sotomayor. The cert petition filed on behalf of Idaho defendant John Joseph Delling had claimed violations of due process and the ban on cruel and unusual punishment. Delling was accused of killing two persons in a 2007 crime spree.

According to Breyer’s opinion, Idaho bars insanity as a defense to criminal conduct, although it does allow defendants to introduce expert evidence to prove they did not have the mental capacity to form intent. “Thus, in Idaho, insanity remains relevant to criminal liability, but only in respect to intent,” Breyer wrote. “The difference between the traditional insanity defense and Idaho’s standard is that the latter permits the conviction of an individual who knew what he was doing, but had no capacity to understand that it was wrong.”

Breyer offered a hypothetical example to show the difference. In case one, the defendant shoots a victim he believes to be a wolf. In case two, the defendant shoots a person he knows to be a human, but does so believing a supernatural wolf issued the order to kill. In Idaho, the defendant could defend a murder charge in case one on the basis that he lacked mens rea, but could not assert an insanity defense in case two.

Breyer cites the arguments of amici that seriously mentally ill defendants often suffer from paranoid delusions that their conduct is justified, just as the hypothetical defendant did in case two. “In view of these submissions, I would grant the petition for certiorari to consider whether Idaho’s modification of the insanity defense is consistent with the 14th Amendment’s due process clause,” he wrote.